A model represent a table, that is, a collection of rows, where
each row is a collection of columns, where one or more of the columns
are used to identify each row from the other ones.
When using toXML methods, column contents will be formatted as
XML, that is, '&', '<' and '>' are translated so XML structure is not
altered by view contents.
This class implements a view with columns mapped to arbitrary names
Notice that real column names always return right values even when
they are mapped to arbitrary names.
A model represent a table, that is, a collection of rows, where
each row is a collection of columns, where one or more of the columns
are used to identify each row from the other ones.